Cathode-Ray Tube

Background

A cathode-ray tube, often called a CRT, is an electronic display device in
which a beam of electrons can be focused on a phosphorescent viewing
screen and rapidly varied in position and intensity to produce an image.
Probably the best-known application of a cathode-ray tube is as the
picture tube in a television. Other applications include use in
oscilloscopes, radar screens, computer monitors, and flight simulators.

The cathode-ray tube was developed in 1897 by Ferdinand Braun of
Strasbourg in what was then the French-German region of Alsace-Lorraine.
It was first used as an oscilloscope to view and measure electrical
signals. In 1908, A.A. Campbell-Swinton of England proposed using a CRT to
send and receive images electronically. It wasn't until the 1920s,
however, that the first practical television system was developed. The
concept for a color cathode-ray tube was proposed in 1938 and successfully
developed in 1949.

Although General Electric introduced their first television set for home
use in 1928, commercial television broadcasting remained an experimental
technology with only limited range and audience. It took until the
late-1940s before television net-works had established themselves
sufficiently to start a boom in consumer sales. Black-and-white television
sets gave way to the first color sets in the 1960s. In the following
decades cathode-ray tubes for televisions got both larger and smaller as
manufacturers sought to satisfy consumer wants. Recent developments have
included tubes with flatter faces, sharper comers, and higher resolution
for better viewing.

A CRT consists of three basic parts: the electron gun assembly, the
phosphor viewing surface, and the glass envelope. The electron gun
assembly consists of a heated metal cathode surrounded by a metal anode.
The cathode is given a negative electrical voltage and the anode a
positive voltage. Electrons from the cathode flow through a small hole in
the anode to produce a beam of electrons. The electron gun also contains
electrical coils or plates which accelerate, focus, and deflect the
electron beam to strike the phosphor viewing surface in a rapid
side-to-side scanning motion starting at the top of the surface and
working down. The phosphor viewing surface is a thin layer of material
which emits visible light when struck by the electron beam. The chemical
composition of the phosphor can be altered to produce the colors white,
blue, yellow, green, or red. The glass envelope consists of a relatively
flat face plate, a funnel section, and a neck section. The phosphor
viewing surface is deposited on the inside of the glass face plate, and
the electron gun assembly is sealed into the glass neck at the opposite
end. The purpose of the funnel is to space the electron gun at the proper
distance from the face plate and to hold the glass envelope together so
that a vacuum can be achieved inside the finished tube.

The CRT used in a color television or color computer monitor has a few
additional parts. Instead of one electron gun there are three—one
for the red color signal, one for blue, and one for green. There are also
three different phosphor materials used on the viewing
surface—again, one for each color. These phosphors are deposited in
the form of very small dots in a repeated pattern across the
screen—red, blue, green,
red, blue, green, and so on. The key to a color CRT is a piece of
perforated metal, known as the shadow mask, which is placed between the
electron guns and the viewing screen. The perforations in the shadow mask
are aligned so that the red gun can fire electrons at only the phosphor
dots which produce the red color, the blue gun at the blue dots, and the
green gun at the green dots. By controlling the intensity of the beam for
each color as it scans across the screen, different colors can be produced
on different areas of the screen, thus producing a color image. To give an
idea of how small the perforations and dots have to be, a 25-inch (63 cm)
color television picture tube may have a shadow mask with 500,000
perforations and 1.5 million individual phosphor dots.

Design

The electron gun must be designed for each new application. New screen
sizes, new overall glass envelope dimensions, and new image resolution
requirements all require a new gun design. Brighter images may require
higher power accelerating coils. Finer image resolution may require
improved beam focusing coils or plates. While the basic design remains the
same, the details are constantly refined.

Likewise the basic design of the phosphor viewing surface is fairly well
defined, but the details may change. New image resolution requirements may
require a new method of depositing the phosphor dots on the face plate,
which in turn may require new material processing techniques. The search
for truer colors may result in new material formulations. The amount of
time the phosphors emit light, or glow, after being struck by the electron
beam is also important and is controlled by the chemical composition of
the phosphor. This property is called persistence. In a color television,
the electron beam scans the screen 25 times per second. If the persistence
is longer than one twenty-fifth of a second (0.04 second), the image would
show two scans at the same time and would appear blurred. If the
persistence is shorter than this time, the image from the first scan would
have disappeared before the second scan came along, and the image would
appear to flicker.

Even the glass envelope requires extensive design. Strength, radiation
absorption characteristics, temperature tolerance, impact resistance,
dielectric properties, and optical clarity are a few of the design
criteria used when designing the glass components. Computers may be used
to perform finite element analysis to evaluate the stresses in complex
envelope shapes. This technique divides the part into a finite number of
smaller, more easily definable pieces, or elements, and then performs the
calculations for each element to spot unacceptably high stress
concentrations. Using the computer, dimensions for contours and wall
thickness can easily be adjusted until a satisfactory design is achieved.

Raw Materials

Cathode-ray tubes use an interesting and varied assemblage of raw
materials. In many cases, it is the raw materials, not the design or
manufacturing process, that determine the performance characteristics of
the finished product.

The electron gun is made from a variety of metal pieces. The cathode, or
electron emitter, is made from a cesium alloy. Cesium is used as a cathode
in many electronic vacuum tube devices because it readily gives off
electrons when heated or struck by light. In a CRT, the cathode is heated
with a high resistance electrical wire. The accelerating, focusing, and
deflection coils may be made from small diameter copper wire. A glass tube
protrudes from the rear of the electron gun assembly and is used to
evacuate the air from the finished CRT.

The phosphor viewing surface is formed from a continuous layer of a single
material in monochromatic CRTs, or is composed of individual dots of three
different materials in color CRTs. Zinc sulfide is a common phosphor
material. The color is determined by adding a very small amount of
material called an activator. Zinc sulfide with 0.01% silver activator
emits a blue light. When a 0.001% copper activator is used, it produces a
green light. A 50/50 mixture of zinc sulfide and cadmium sulfide with a
0.005% silver activator produces a yellow light. Red light can be produced
by adding silver or copper to zinc sulfide mixed with a

A CRT consists of three basic parts: the electron gun assembly, the
phosphor viewing surface, and the glass envelope. The electron gun
assembly consists of a heated metal cathode surrounded by a metal
anode. The phosphor viewing surface is a thin layer of material which
emits visible light when struck by an electron beam. The glass
envelope consists of a relatively Rat face plate, a funnel section,
and a neck section.

high percentage of cadmium sulfide. The phosphors are usually ground into
a fine powder before they are applied to the inside of the face plate.

The glass envelope uses slightly different raw materials for each of its
three component parts. The basic raw material for all of the glass
components is silica. Alumina may be added to adjust the flow properties
of the molten glass when forming it. Various oxides are used to lower the
melting temperature. Barium oxide, strontium oxide, and lead oxide are
used to provide radiation protection in the neck and funnel. The face
plate, on the other hand, must have a minimum of lead oxide to prevent a
discoloration phenomenon known as electron or x-ray browning. Neodymium
oxide may be used on the face plate to enhance the contrast of the viewed
picture.

In color CRTs, the shadow mask is usually made from a thin sheet of a
nickel alloy.

The Manufacturing
Process

The glass envelope or its components are usually formed at a glass
manufacturing facility and shipped to the cathode-ray tube manufacturer
who forms the phosphor viewing screen, fabricates and assembles the
electron gun, and assembles the finished CRT.

Forming the glass envelope

1 The glass ingredients are weighed and mixed prior to melting. The
glass is melted in gas-fired furnaces about 500-3,000 square feet
(46-279 sq m) in size. If this is a continuous process, new ingredients
are added to maintain a constant level as the molten glass flows out of
the furnace to the forming areas. Before forming, the molten glass must
be cooled somewhat and made uniform in temperature throughout.

2 The face plate is normally pressed into the desired shape by dropping
a gob of molten glass into a mold and pressing on the gob with a
plunger. The funnel can be formed either by pressing or by centrifugal
casting. In the casting method a gob of molten glass drops into a mold,
which then spins rapidly to spread the glass uniformly over the inside
surface of the mold. A grooving disk near the top of the mold cuts the
soft glass at the desired height so that the excess glass can be removed
easily. The neck is made from glass tubing, and one end is flared to
facilitate insertion of the electron gun.

3 In a monochromatic CRT the three glass components are joined together
before they are shipped to the CRT manufacturer. In a color CRT only
the neck and funnel are joined, and the face plate is shipped separately
for further processing. The glass components are usually joined by
heating the mating surfaces to a high temperature with gas jets or
electric heaters.

Applying the phosphors

4 In monochromatic CRTs the phosphor viewing surface is coated on the
inside of the glass face plate. This is done by preparing a liquid
suspension of the phosphor and pouring a measured amount into the neck
of the glass envelope along with a gelling agent. After about 20
minutes, the coating has set and the excess liquid is poured off. The
process for color CRTs is more complicated. First the shadow mask is
made by applying a light-sensitive coating to the thin mask material,
exposing it to light through a perforated template, and then etching
away the exposed coating with an acid to form the millions of holes. The
mask is then pressed into a slightly curved shape and attached just
behind the face plate. The face plate is placed in a centrifuge and the
inside surface is coated with the green phosphor material. The
centrifuge spins the face plate to ensure an even coating of phosphor. A
strong ultraviolet light is shown through the mask to harden the green
phosphor material into hundreds of thousands of dots. The remaining
material is then washed off. This process is repeated to form the red
and blue phosphor dots, with the ultraviolet light being shifted a small
amount each time. When this process is finished, the glass face plate is
joined to the funnel. On color tubes, the phosphor dots are sensitive to
high temperatures, so instead of using high-temperature gas jets, a
mixture of chemical solvent and powdered glass, called a frit, is
applied to the joint. This acts like a glass "solder," and
the joint can be sealed at a much lower temperature.

Assembling the electron gun

5 The metal components of the electron gun are precision formed. If
coils are used they are wound from fine copper wire. Some electron guns
use metal plates instead of coils, and these plates are stamped and
formed. The components are assembled either by hand or with automated
machines in a clean environment. The glass tube is sealed into the base,
and the base is welded into the gun assembly.

Final assembly and packing

6 The inside of the glass envelope neck is lubricated with graphite, and
the electron gun is inserted and aligned. The neck is then sealed around
the gun. A vacuum pump is attached to the glass tube extending from the
rear of the gun, and the inside of the CRT is evacuated of air. When the
proper vacuum has been achieved, the glass tube is heated and quickly
pinched closed to form a seal.

7 The finished CRT is tested for performance and carefully packed to
prevent damage. Because the CRT is under a high vacuum, any fracture in
the glass envelope could result in an inward explosion known as an
implosion.

Quality Control

Although the operating principle of a cathode-ray tube is simple, the
manufacturing process requires strict controls and precise alignments. The
phosphor materials must be extremely pure to achieve the desired colors.
Even a tiny variance in the amount of activator used can result in a
significant change in color. Likewise, when you consider that a color
television CRT requires the placement of over a million tiny dots side by
side on the viewing surface, even a small error in alignment could be
disastrous.

Byproducts and Recycling

The principal byproduct of CRT manufacturing is scrap glass. Much of this
glass is recycled. Recycled glass with a high content of lead oxide is
used to provide radiation protection in CRT funnels and has completely
replaced previous sources of lead oxide for this application.

The Future

The worldwide market for cathode-ray tubes was estimated at nearly 400
million
units in 1994 and is expected to grow at a 6% annual rate through 2000.
The color television market is expected to grow at a 5% annual rate, while
the color computer monitor market is expected to grow at a 20% rate. In
the television market, the demand for larger television picture tubes with
higher image resolution is expected to continue.

One important trend is the development of high definition television
(HDTV), which has scanning rates more than twice that of conventional
systems. This will require new electron gun designs as well as new glass
materials and technologies to handle the doubled radiation rate.